This invention relates to improvements in security devices and more particularly to improvements in electronic detection devices working on the Doppler principle. The invention is applicable to security devices utilising the Doppler principle in which the detection technique employs quadrature mixers to discern between positive and negative Doppler shifts. This technique is common among ultrasonic and radio frequency type Doppler detection devices.
Electronic detection devices utilising the Doppler principle have been widely used for some years. Such devices detect a change in received frequency from transmitted frequency, the difference being the Doppler shift. These devices should ignore spurious signals generated by external environmental influences which alter the received frequency and/or phase, but do not always do so thus resulting in the generation of false alarms.
In the case of frequency discriminating devices of the above type the detector may not be able to discern between (a) small oscillatory noise or vibration sources which cause phase jitter on the received signal and that fall within the pass-band of the receiver and which when combined with the appropriate mean phase of the received signal cause false alarms, and (b) real signals producing either a positive or negative Doppler shift caused by a moving body within the field of the devices.